Historic Range of Variability in Landscape Structure

and Wildlife Habitat


San Juan National Forest


January 2005



Principal Investigators:

            

Kevin McGarigal

Department of Environmental Conservation, University of Massachusetts, Amherst, MA01003


William H. Romme

Department of Forest, Rangeland, and Watershed Stewardship, Colorado State University, Fort Collins, CO 80523


Technical Assistants:


David Goodwin, Erik Haugsjaa, Eduard Ene, and Brad Compton

Department of Environmental Conservation, University of Massachusetts, Amherst, MA01003


Dan Kashian

Department of Forest, Rangeland, and Watershed Stewardship, Colorado State University, Fort Collins, CO 80523



      The San Juan National Forest (SJNF) occupies the southwestern portion of the South Central Highlands Section of the Southern Rocky Mountains Province in southwestern Colorado (Figure-study area map). The SJNF is a montane landscape that rises from the surrounding basins (ca. 1,500 m) to elevations as high as ca. 4,200 m. Local climate and vegetation change dramatically along this elevational gradient, from desert scrub and grasslands in the semi-arid basins to spruce-fir and aspen forests and alpine tundra at the highest elevations. The SJNF has provided important timber, forage, and water resources, as well as recreation and aesthetics, to the inhabitants of western Colorado for over a century. Looking to the future, public land managers representing the SJNF are partnering with the public to develop a long-range vision for sustainable management of the SJNF, in compliance with the requirements of the National Forest Management Act of 1978. As a component of this planning process, we developed a suite of computer models that simulate changes in landscape patterns and wildlife habitat under a range of natural and anthropogenic disturbance regimes. This report documents the results of the first phase of this modeling effort. Specifically, we applied these models to characterize the pre-1900 range of variability (hereafter referred to as the “historic range of variability” [HRV]) in landscape structure and wildlife habitat on the SJNF. The purpose of this report is three-fold:

 

(1) to provide an overview of the vegetation patterns and dynamics of the SJNF for general understanding and planning purposes;

 

(2) to describe the models used in this study, including the empirical basis for their parameterization and application; and

 

(3) to present the results of our HRV analyses and discuss their management implications; specifically, to describe the HRV in landscape structure and wildlife habitat.


      Our HRV results have at least three basic uses: (1) to improve our general understanding of landscape dynamics; (2) to use as a reference or benchmark for evaluating the state of the current landscape (i.e., to determine to the degree of “departure” from HRV conditions); and (3) to use as a reference or benchmark for comparison with alternative management scenarios. Our study was designed and implemented with #1 and #3 in mind; in fact, we strongly believe that the most appropriate and defensible use of our quantitative findings is in the context of evaluating the relative impacts of alternative scenarios; specifically, comparing the relative impacts of alternative management scenarios against a common reference or benchmark. However, as this project neared completion it became clear that our results were increasingly being sought for use #2; specifically, as the basis for evaluating current departure from HRV in the Fire Regime Condition Class mapping project. For this reason, we have included in the results and discussion a substantial section on HRV departure.


      This report is intended to complement the detailed landscape condition analysis completed for the South Central Highlands Section, southwestern Colorado and northwestern New Mexico (Romme et al. 2002). As such, we made every effort to avoid redundancies. Here, we focus exclusively on the results of the computer modeling exercise. In this report, for example, we provide only a brief description of each vegetation type; a more detailed description of each vegetation type can be found in the previous report, including a qualitative description of reference period conditions, a description of past human land use legacies, and a discussion of management opportunities and constraints. Here, we provide a detailed quantitative analysis of the simulated vegetation dynamics under the historic reference period conditions to complement the detailed, but qualitative, landscape condition assessment of the previous report. Furthermore, here we focus exclusively on the SJNF, whereas the previous report provided a general description for the entire South Central Highlands.


      This report is organized into several major sections [click on the corresponding links] as follows:

 

          Executive Summary - This section provides a concise (10-page) summary of the project, including a brief overview of the major conclusions. Note, click here for a pdf version of the executive summary, with figures and tables included, that is suitable for printing).

 

          Justification - This section provides a brief background and the conceptual basis and motivation for this study.

 

          Scope and Limitations - This section provides a brief description of the scope and limitations of this study. All results and conclusions are subject to the constraints discussed in this section.

 

          Project Area - This section provides a brief description of the project area.

 

          Methods - This section provides a general description of the methods employed in this study, including a detailed description of the computer models (RMLANDS, FRAGSTATS, and HABIT@), spatial data, model parameterization, and statistical analyses. In particular, we describe each cover type, including its floristic composition, geographic distribution within the project area, stand conditions, and major succession and disturbance pathways, and detail the parameterization of each disturbance process. We also provide a general description of several habitats of special interest and the selected wildlife indicator species. In addition, we describe our approach for examining the effects of scale (i.e., landscape extent) and context (i.e., geographic location) on HRV, and our approach for determining the degree of HRV departure (i.e., Fire Regime Condition Class).

 

          Results - This section includes the detailed HRV results pertaining to each disturbance process, cover type, habitat of special interest, and wildlife indicator species. In addition, we present the results of analyses aimed at examining the effects of scale (i.e., landscape extent) and context (i.e., geographic location) on HRV assessments.

 

          Discussion & Conclusions - This section includes a discussion of the key results of this study and the major conclusions relevant to land management.

 

          Literature Cited - This section includes a complete listing of all citations.